The present invention relates to radars and sonars. It is known practice to improve the distance resolution of a radar or a sonar by the technique of pulse compression. Indeed, the distance resolving power .DELTA.D of a radar or a sonar is related to the duration .tau. at reception of the pulse waveform transmitted by the relation: ##EQU1## where c is the wave propagation speed. Since the duration .tau. and the width of the frequency spectrum or passband .DELTA.F of the pulse waveform transmitted are related by a relation of the form: EQU .tau..DELTA.F=.beta.
it is also possible to state that the distance resolving power of a radar or a sonar is inversely proportional to the passband of its pulse waveform.
The technique of pulse compression consists in lengthening the pulse waveform on transmission and then in compressing it on reception, thereby limiting the peak power to be transmitted. To put it into practice, use is generally made, at transmission, of a linearly frequency-modulated quasi-rectangular pulse and at reception, of a compression filter which delays the various frequency components of the pulse differently so as to make them coincide. The degree of compression and hence the distance resolving power is limited by the capacity available for producing large-band frequency-modulated pulses.
One way of countering this limitation consists in employing a particular waveform known by the designation: "synthetic band" (otherwise known as Stepped Frequency) and described in particular in the book by J. P. Hardange, P. Lacomme, J. C. Marchais, entitled: "Radars aeroportes et spatiaux" [Airborne and space radars], published by Masson 1995, pages 165-167.
Synthetic band consists in transmitting a waveform composed of a repetitive pattern of N successive pulses of duration T, of passband B, spaced apart by an interval .DELTA.T in time and .DELTA.F in frequency, the first being centred on f.sub.0, the second on f.sub.0 +.DELTA.f, the third on f.sub.0 +2.DELTA.f, etc.
After demodulation by its carrier frequency, each pulse received is filtered by a matched filter and then sampled and converted into digital. The processing continues with a discrete Fourier transform on N samples belonging to the same distance gate and acquired in succession for the N transmission frequencies of the N pulses of the waveform transmitted.
The response .vertline.c(.DELTA.t).vertline. of the receiver matched to the waveform, to the echo returned by a target after a time t.sub.0, which results from the discrete Fourier transform, corresponds to that of the filter matched to each elementary pulse multiplied by a function similar to a sinc: ##EQU2## It has a 3 dB width of: ##EQU3## and contains a periodic term of period 1/.DELTA.f.
The resolution is fixed by the width 1/N.DELTA.f of the reception spectrum of the pattern of the waveform transmitted whereas it would be only 1/B for a transmission waveform pattern limited to an elementary pulse.
To comply with the sampling theorem, the sampling period .tau..sub.e must be less than the inverse 1/B of the band of the elementary pulses of the waveform. Moreover, to avoid distance ambiguities, the analysis performed by the Fourier transform must cover at least the m distance gates over which the response of a target to a pulse at the output of the matched filter extends, this being conveyed with regard to the elementary gap .DELTA.f between the carrier frequencies of the pulses, the sampling period .tau..sub.e and the passband B of each pulse by the condition: ##EQU4## This condition imposes some overlap between the frequency spectra of the elementary pulses which implies that the distance resolution is improved by only a factor N.DELTA.f/B which is less than N.
Another counterargument to the use of N successive pulses in the pattern of the waveform transmitted is that the coverage of a given swathe, with a certain resolution, takes N times as long as for a conventional pulse compression radar. If the constraint is to comply with a minimum recurrence frequency as in the case of a mapping radar, the dimension of the swathe is reduced in a ratio N.